What is actually wrong with the thread, is it A) turning still after you fully do it up or B) starting in the thread and then jamming?
If it is A then you need to Helicoil it, if it is B you may be able to retap the thread, or you may be able to go to a 1/2" fine if you can find a sump bolt out of a old school car (wreckers)
The thread is a 12mmX1.25 pitch which is a metric fine, I hope this helps but I dont have a tap to suit, I can buy one though